Glass products have long been made from a pre-formulated feedstock (also sometimes termed a glass batch) that is charged into a glass furnace and melted to produce molten glass for subsequent formation into the desired glass product. A typical feedstock includes a physical mixture of virgin raw materials and, optionally, recycled glass materials known in the industry as “cullet.” The virgin raw materials can contain quartz sand (crystalline SiO2) and other ingredients, such as soda ash (Na2CO3) and limestone (CaCO3) for soda-lime-silica glass, for example, and the cullet primarily contains shards of glass from previously-formed consumer or commercial glass products. The cullet component of the feedstock can vary based on the glass-forming process being practiced and the desired characteristics of the final glass product (e.g., color, transparency, etc.). In many instances, however, the feedstock may contain up to about 80 weight percent cullet, with the remainder being virgin raw materials which may or may not include, in addition to the ingredients listed above, a small percentage of other ingredients including glass network formers, network modifiers, colorants, decolorants, fining agents, and redox agents, to name but a few.
The residence time of the conventional glass feedstock in the glass furnace is relatively long. This can be attributed to several factors. First, the largest component of the virgin raw materials, quartz sand, and usually some of the other virgin raw material ingredients—e.g., soda ash and limestone for soda-lime-silica glass—are crystalline materials. Their crystal structures, including intermediate crystalline phases, are generally present up to about 1200° C., as melting and dissolution of these materials does not occur instantaneously. Second, the glass feedstock needs to be dispersed and homogeneously mixed by convection after being melted to produce molten glass, which is a time-consuming process. Quartz sand, in particular, takes the longest to disperse on account of its slow dissolution rate and the tendency to agglomerate into SiO2-rich regions within the glass melt known as “cord.” The presence of cord is indicative of glass inhomogeneity and may result in imperfections or defects in the finished glass product. Third, some of the virgin raw material ingredients—e.g., soda ash and limestone for soda-lime-silica glass—are carbonate-containing materials that, when melted, release carbon dioxide (CO2). The evolution of carbon dioxide during feedstock melting introduces bubbles in the resultant molten glass, which, in turn, can cause a thin spot or bubble defect in the finished glass product. Any such bubbles are typically removed from the molten glass in a process known as “refining the glass.” To address the challenges associated with melting and homogenizing crystalline raw materials and to remove bubbles caused by carbon dioxide evolution, among other factors, conventional glass feedstocks are usually subjected to high temperatures and heating times of 24 hours or more in the glass furnace in order to obtain suitably-refined and chemically homogenized molten glass.
The melting of the glass feedstock can be made less taxing if some of the virgin raw materials are replaced with cullet in the feedstock. The cullet accelerates the melting of the feedstock and lowers furnace energy consumption as compared to a feedstock that contains all virgin raw materials. Cullet has this effect because it has already been melted, mixed, and formed into a glass product and will not release carbon dioxide when re-melted since it is not an intrinsic carbonate-containing material. But cullet is not widely available as a commodity in some regions and, even if it is, bulk purchases of the recycled material are subject to great variations in color and other characteristics that may restrict glass manufacturing options. Post-consumer cullet also has the tendency to be contaminated with metals, glues, and other organics, and is sometimes difficult to uniformly mix with virgin raw materials in the glass furnace when melted. Moreover, even with the addition of cullet, current glass manufacturing practices still typically involve melting the glass feedstock and homogenizing/refining the molten glass in the glass furnace at a temperature of around 1400° C. or higher for at least about 24 hours. Such long processing times at elevated temperatures require a lot of energy and slow the overall glass-making process. Additionally, these long processing times limit further processing options for the molten glass.